Vegetable oils, such as rapeseed oil, are increasingly being considered as renewable fuel sources providing an alternative to fossil fuels.
Such oils can to be extracted from the seed material (oilseed) using mechanical presses (often referred to as expellers), chemical processes, or a combination of both. The chemical process (solvent extraction) is highly efficient but capital intensive and it is also considered unsafe due to the use of flammable chemical solvents. Solvent extraction is used in operations that process many tons of oilseed per hour, while mechanical presses are used for processing oilseeds in the order of kilograms per hour up to several hundreds of kilograms per hour.
Mechanical presses are quite simple in construction, but far less efficient in terms of oil extraction when compared to solvent extraction and, as a result, a large percentage of the vegetable oil is left in the press cake (the solid residue after the pressing process). Typical residual oil content in the press cake from modern commercial expellers is between 8% and 12%. The residual oil is considered a financial loss to an oilseed processer as it normally does not add to the monetary value of the press cake (typically used as animal feed). Therefore increasing the efficiency of a mechanical press can increase the profitability of a small to median size vegetable oil extraction operation.
Mechanical presses for the recovery of oil from oil seed, otherwise known as expellers, are typically used for recovering vegetable oils in two ways;
a) as a high pressure operation leading to maximum oil recovery and consequently low residual oil in the press-cake, or
b) as a pre-press operation prior to solvent extraction.
In a pre-press operation, the expeller operates at a relatively low pressure in order to produce a press-cake with high porosity to facilitate the solvent percolation during the follow up solvent extraction. Therefore, maximum oil extraction is not the main goal of a pre-press operation. In a pre-press operation, the press-cake leaves the expeller with a residual oil content of about 20% by weight.
However, in the full press operation, the aim is to extract the maximum amount of the available oil in the oilseed. Therefore, in the full press operation, the expeller operates at a relatively high pressure in order to produce a press-cake with the minimum amount of residual oil therein.
A typical expeller generally comprises a screw auger rotatably mounted within a cylindrical expeller barrel. The expeller is typically divided into three sections, namely a feed section, a compression section, and a discharge section.
The feed section is at the beginning or root end of the screw auger and incorporates an opening in the side wall of the expeller barrel into which seeds can be gravity fed on demand, or in some cases, under pressure by an auxiliary feed gear (force fed expellers). In the feed section, the screw auger transports the seeds towards the compression section.
In compression section the screw auger is shaped to compress and break up the cell walls of the seeds to extract the oil therefrom. The expeller barrel includes a draining area were the oil can flow out of the expeller barrel via oil outlet channels formed in the side wall thereof. In such prior art expellers, the draining area is typically at or adjacent the discharge section of the expeller.
The discharge section includes a press cake outlet, and is commonly defined by an expeller die mounted on or integrally formed with a discharge end of the expeller barrel. The expeller die comprises narrowing tapered inner walls having a relatively narrow outlet opening at an end (known as a die land) thereof through which the press cake is extruded.
During operation of the expeller, a column or plug of compressed meal (press cake) is formed in the discharge section of the expeller, while new seed material is rammed into the compression section by the action of the screw auger in the feed section. New cake is constantly formed at the inner end of the discharge section as the pressed cake is constantly discharged through the outlet opening of the discharge section. The operation may proceed continuously by a constant addition of seed material at the feed section.
The shape of the screw auger has to be designed in a way to be able to cause a higher volume displacement at the feed section compared to the volume displacement at the discharge section, such that the material is compressed as it is conveyed down the expeller barrel. The seed material is subject to increasing axial and radial pressure as it is conveyed from the feed section to the discharge section and the resulting pressure causes the oil to be expelled from the oilseed cells. The expelled oil exits the expeller barrel via the oil outlet channels in the draining area adjacent the discharge end of the expeller barrel.
Various attempts to improve the oil recovery efficiency of mechanical expellers have been made in the past by academic researchers (Vadke & Solsulski, 1988, Isobe et al, 1992, Dufaure et al., 1999, Singh & Bargale, 1999, Kartika & Rigal, 2005, Olayanju et al, 2006, Mpagalile et al, 20007, Evon et al., 2007, Voges et al, 2008, Singh et al, 2010, Deli et al 2011) and by the expeller manufactures themselves. Most of the developments have been concentrated in the design of the expeller screw. Attempts to improve the expeller efficiency have been made by changing the screw configuration (single stage, double stage, worm design, etc.) or by adding an extra counter rotating screw (twin screw expellers).
An object of the present invention is to provide a screw press and method of operation that overcomes the problems of the prior art and maximises oil extraction.